It is intended that the term “computer” includes all processor controlled devices, which may be various portable devices, such as laptop computers, tablets, smart phones or other portable devices, as well as desktop and other less portable computing devices. The desktop and other computing devices are coupled to a separate display monitor. Many portable devices, as well as having an integrated display screen, can also be coupled to such a separate display monitor, as well as to local displays.
In the past, display monitors were formed by cathode ray tubes, in which a display signal was rastered across the screen line by line. In other words, a single electron beam was scanned from one side of the screen to the other to form a line of the display. The beam was then returned to the first side of the screen and scanned across to form the next line of the display, and so on. Therefore, the display signals that were generated by the computer for display, were in raster format, providing each line, one at a time. As display monitors evolved to flat screen LCD, OLED and other display technologies, the display signals remained in raster format, partly for backward compatibility and partly because the monitors were designed to display such raster signals.
Many of the newer monitors are formed, however, by pixel groups (of RGB pixels) that are addressable using row and column addressing, so that each pixel group is individually addressable. Nevertheless, the incoming display signals are still provided by the computer device in raster format, whether compressed or not, and the monitor then processes the raster display signal to be displayed on the monitor.
Therefore, an image is displayed on the monitor by controlling the display for all the pixel groups in a line for all the lines, in turn. The image is therefore refreshed each time the monitor displays all the new lines of the image. This is normally done at 60 Frames Per Second (FPS) whether or not the image changed as that is the rate that was required for smooth display on electron beam based displays.
When several different applications are operating at once, for example, when different windows are open at once in a computer system, there may be several different images, for example, a text document, a picture, web browser with text and still and/or moving images, and perhaps a movie, all open in windows that need to be shown and refreshed in a single “image” shown on the display at the same time. Each application generates its own output and an image compositor takes each of those outputs and uses them to form a combined image according to the positions of the windows to be displayed on the monitor. The combined image is updated and stored in a frame buffer and the stored complete image is then transported to the monitor and displayed on the screen by taking all the image data on a line-by-line basis from the frame buffer and displaying them in a rastered manner.